1. Field of the Invention
The present invention relates to a dental handpiece control device, and more particularly to a dental handpiece control device including a pressure regulating device or a flowrate regulating device to control the speed of an air turbine by changing the back pressure of the air turbine in a leakless exhaust passage which discharges air spent to drive the air turbine into atmosphere.
2. Prior Art
An air turbine handpiece has long been used particularly in dental treatment to remove decayed portions by high-speed cutting or to improve the surface rougness of teeth after abutment formation and to form margins by low-speed cutting. For low-speed control, a known handpiece with an air turbine uses a control device disposed in an air supply passage as shown in FIG. 12 for example, as disclosed in Japanese Patent Provisional Publication No. 57-6939 and Japanese Utility Model Provisional Publication No. 59-15618. More specifically, compressed air from a compressed air source 410 passes through a filter 420 and the pressure of the compressed air is reduced by a fixed-amount pressure regulating valve 430 to the pressure specified for a handpiece. The compressed air passes through an air tube 4 and is supplied to a pressure regulating valve or a flowrate regulating valve 440. The pressure of the compressed air is further reduced by the valve and supplied to an air turbine handpiece 450. When the regulating valve is fully opened, the pressure set by the pressure regulating valve 430 is supplied. The flowrate regulating valve 440 is generally controlled by a foot pedal so that the dentist can obtain the speed suited for treatment.
A dental handpiece having a construction to rapidly stop the air turbine has been disclosed by Japanese Patent Provisional Publication No. 46-31519 and Japanese Utility Model Provisional Publication No. 56-124413. The former has two 3-way solenoid valves and a circuit delay relay. In case of stopping the air turbine by controlling a foot switch, compressed air to the air passage is stopped by one of the 3-way solenoid valves and compressed air begins to be supplied through the other solenoid valve from the compressed air source to the exhaust passage to brake the turbine. After the turbine is stopped, the relay functions so that the air supply passage which was shut off by the valve is opened to discharge the remaining air to atmosphere after the lapse preset time. The latter is an improvement of the former, that is, fluctuation of delay operation due to the deterioration of the relay or changes in temperature and humidity is reduced so that the turbine can be stopped in the minimum time.
Referring to FIG. 13, an air timer 315 is used in instead of the delay relay. When stopping is begun by releasing a foot switch 320, a four-way solenoid valve 302 is switched to supply compressed air from a compressed air source 301 to the exhaust passage 317 of a handpiece 300 and to brake the turbine of the handpiece. At the same time, the air supply passage 316 is opened to atmosphere. After a specified time, a pilot valve 310a is activated to stop supplying compressed air to the exhaust passage 317 as shown in FIG. 13. As a result, compressed air is jetted to the rear of the turbine so that compressed air can be supplied to the bearing mechanism even when the turbine is braked. This prevents dust from entering the bearing mechanism due to buildup pressure in the mechanism. However, these prior arts have the following defects.
As detailed in the hydrodynamic analysis description below, when the turbine speed is intended to be lowered by reducing the air supply pressure at the air supply passage, exhaust air from the turbine expands until the pressure of the exhaust air reaches atmospheric pressure with almost no resistance and is discharged. As a result, it is impossible to control the pressure of high-speed air jetted from the turbine nozzle and thus the rotation torque of the turbine decreases drastically. Therefore, turbine speed control by regulating air supply pressure is not suited for dental treatment which needs a relatively high rotation torque at low speed. Furthermore, the load capacity and rigidity of the bearings decrease since air supply pressure is reduced to rotate the turbine at low speed in the conventional handpiece with an air turbine of a pneumatic bearing type.
Moreover, since compressed air for braking is supplied from the exhaust passage and discharged from the air supply passage during braking, the air pressure in the air supply ports for the bearings drops when the air supply pressure drops. As a result, the bearing clearance in the thrust direction is not properly obtained and metal contact is caused. Moreover, since compressed air is supplied from the exhaust passage, the bearing clearance in the radial direction becomes eccentric and metal contact is also caused. Thus, wear of parts is hastened. The clearance in the radial direciton used to regulate the rotation center of the turbine is a very small value of 5 to 15.mu., and the size of the clearance is determined depending on the shapes of the air supply ports and bearings. Since the dimensional allowance of the clearance is very small (high-speed of 500,000 rpm is obtained when the inner diameter of the air supply ports is 0.18 mm, and the clearance in the radial direction is 10.5.mu.), the bearing performance is significantly lowered eve if the handpiece parts are worn only 1 to 2.mu.. When the handpiece is used after the handpiece parts have been worn, the turbine speed drops and greater noise is generated.